Modern aircraft generally possess synthetic vision system (SVS). This system allows the crew to be presented with synthetic image of the exterior landscape generally comprising piloting or navigation information.
An SVS system comprises a cartographic database representing the overflown terrain, a geolocation system, electronic computing means and one or more visualisation devices embedded in the cockpit of the aircraft. The geolocation system is of GPS (global positioning system) type. It may be coupled to the inertial system of the aircraft. The overall global positioning system delivers at least the following parameters: the position of the aircraft in terms of latitude, longitude and altitude, and the orientation of the aircraft in terms of pitch, roll and heading.
Generally, the images displayed on the viewing screens which are located on the front of the instrument panel of the aircraft. The image is a three-dimensional view of the exterior shown so as to be as realistic as possible. The point of view displayed is that along the axis of the aircraft. The synthetic image is computed up to a certain distance away from the aircraft so as to limit the computations required for the display. This distance is referred to as the SVS range. Its order of magnitude is 40 nautical miles. Specifically, beyond a certain distance, the dimensions of the image of the landscape are small. Furthermore, it is of no more than minor interest to the pilot of the aircraft.
This synthetic image generally comprises a piloting and navigation assistance symbology. It conventionally comprises an artificial horizon giving the altitude of the aircraft and indicators giving the altitude and the speed of the aircraft. This symbology also displays a line representing the zero longitudinal altitude indicator, also referred to as the ZPRL (zero pitch reference line). The ZPRL is often, although incorrectly, referred to as the “horizon line”.
As shown in FIG. 1, which represents a vertical cross-sectional view of an aircraft A flying over a terrain T, the ZPRL forms a first angle α with the true horizon line LH. This line forms a second angle β with the limit of the cartographic representation RC which is necessarily greater than the first angle. These angles are generally of several degrees.
FIG. 2 shows the display, on an aircraft visualisation device, of a cartographic representation of the overflown terrain comprising a piloting symbology. This symbology comprises a ZPRL. In this figure, the difference between this ZPRL and the end of the cartographic representation is notable. It has been demonstrated that a significant angular deviation between the SVS and the ZPRL is very confusing for pilots since it does not correspond to the usual visual orders of magnitude. This occurs above all at high altitude, the deviation increasing with the altitude of the aircraft.
Furthermore, certain aeronautical standards such as the AC20-167 standard entitled “Airworthiness Approval of Enhanced Vision System, Synthetic Vision System, Combined Vision System, and Enhanced Flight Vision System Equipment” mandate that the information provided by the SVS is correlated to the ZPRL. For example, the terrain which is located above the altitude of the carrier at a given instant in time must always appear above the ZPRL, if it is close enough to be dangerous.
One means of solving these various problems is to display a cartographic representation over longer distances. The drawback of this solution is a substantial additional need in terms of the performance of the electronic platform, both the central processor and the graphical computing means, in order to allow the image to be displayed at the expected frequency and image quality.